Imprecise FBX Animation
I've encountered this issue myself, I'm not sure if it's Blender or the FBX format, there are things you can do to help mitigate the problem by adjusting the "Sampling Rate" value or "Simplify" value, there are also settings in Unity that may help as well try changing the Keyframe Compression to "Optimal" or "Off" in the Inspector tab. Other than this I haven't found any other solutions myself.